(a) Illustration of mechanism of SPPM-induced ORS contrast, and (b) a schematic representation of SPPM with different PEG corona lengths but the same Fe3O4 core. In ORS contrast, an off-resonance RF saturation pulse only affects the SPPM-containing water protons. Due to fast water diffusion and long RF duration time, a steady-state is reached with lower signal intensity.

T1 (top) and T2 (bottom) relaxivities (r1 and r2) of SPPM with different PEG corona lengths. The difference in r1 values across all formulations is not statistically significant. The single and double asterisks (* and **) indicate statistical significance (P < 0.05) between the SPPM groups of interest. r2 values of SPPM with 2 kD and 5 kD PEG are statistically equivalent (***, P > 0.05).